1. Field of the Invention
The present invention relates to digital communications, and more particularly to a method and apparatus for determining rate of data transmitted at a variable rate in a variable-rate communications system.
2. Description of the Related Art
In variable-rate communications systems such as a variable-rate voice coder, the data rate of transmitted data should be determined in order to receive and process the transmitted variable-rate data without an indication of the data rate. U.S. Pat. No. 5,566,206, entitled “Method And Apparatus For Determining Data Rate of Transmitted Variable Rate Data in a Communications Receiver”, will now be briefly described with reference to FIG. 1.
In a conventional apparatus for determining data rate shown in FIG. 1, a first Viterbi decoder 101 performs Viterbi decoding on received data under the assumption that the data is received at a full rate, a first encoder 102 re-encodes the output from the first Viterbi decoder 101, a first comparator (COMP) 103 compares the received data 123 to the data 128 which is obtained by re-encoding the output 124 from the Viterbi decoder 101 in the first encoder 102, and a first counter 104 outputs the number of errors 129 according to the output from the first COMP 103.
A first summer 105, a second Viterbi decoder 106, a second encoder 107, a second COMP 108 and a second counter 109 perform the same operations as above at a half rate. A second summer 110, a third Viterbi decoder 111, a third encoder 112, a third COMP 113 and a third counter 114 operate as above at a ¼ rate. A third summer 115, a fourth Viterbi decoder 116, a fourth encoder 117, a fourth COMP 118 and a fourth counter 119 operate as above at a ⅛ rate. A microprocessor 122 determines the rate of the received data using error metrics 129, 132, 136 and 140 from the first, second, third and fourth counters 104, 109, 114 and 119, CRC results 125 and 127 from first and second cyclic redundancy code circuits (CRCs) 120 and 121, and Yamamoto Quality Metrics 141 and 142 from the third and fourth Viterbi decoders 111 and 116.
The conventional rate determination apparatus shown in FIG. 1 adopts a post-data rate determination method using Viterbi decoders which have a complicated structure and require much time to decode data. That is, after performing Viterbi decoding at each receivable data rate, the rate of the received data is determined using symbol error rates which indicate the quality of respective decoded data. Thus, power consumption increases due to the complicated structures and processing speed of the Viterbi decoders, in addition to a temporal loss.
In another data rate determination apparatus disclosed in U.S. Pat. No. 5,627,845, entitled “Variable Rate Communication Method and a Variable Rate Communication System,” the rate of received data is estimated using repetition characteristics of the received data. Also, in U.S. Pat. No. 5,689,511, entitled “Data Receiver For Receiving Code Signals Having a Variable Data Rate,” the number of repetitions is determined by calculating an auto-correlation function of a received data frame, and the rate of the received data is then estimated.